By 1915, the CB&Q had 220 "Mikados" on its roster or on order and still needed more motive power. It would purchase 148 more between 1915 and 1923 similar to the Class O-1s.from Baldwin. These locomotives were designated as Class O-1a.and assigned road numbers beginning with the 5060 and running through 5147 for those delivered by 1922. There were not enough numbered in the 5100 series for the last batch of sixty, which were delivered in 1923, so they were assigned road numbers 4940 through 4999. (Wikipedia)

2 versions of the O-1a are now available for reserve. The freight tender (Straight Sided) version is fully reserved. The switcher version, which is far more popular is only available now for reserve.

Including: They are numbers #4960 (Worthington), and #4978 (Elesco). #4978 is currently on display at Union Depot Railroad Museum in Mendota, IL.